U.S. patent application number 13/817541 was filed with the patent office on 2013-07-04 for ultrasound probe and ultrasound diagnostic device using same.
This patent application is currently assigned to HITACHI MEDICAL CORPORATION. The applicant listed for this patent is Tatsuya Nagata, Akifumi Sako, Yasuhiro Yoshimura. Invention is credited to Tatsuya Nagata, Akifumi Sako, Yasuhiro Yoshimura.
Application Number | 20130172750 13/817541 |
Document ID | / |
Family ID | 45605271 |
Filed Date | 2013-07-04 |
United States Patent
Application |
20130172750 |
Kind Code |
A1 |
Yoshimura; Yasuhiro ; et
al. |
July 4, 2013 |
ULTRASOUND PROBE AND ULTRASOUND DIAGNOSTIC DEVICE USING SAME
Abstract
In order to obtain a high-resolution ultrasound diagnostic image
while reducing the back side reflection of a ultrasound irradiated
to the side opposite to the ultrasound transmission direction of an
ultrasound transmission/reception device, disclosed is an
ultrasound probe, wherein a substrate is provided thereon with a
cavity, insulation layers having the cavity therebetween, and an
upper layer electrode and a lower layer electrode having the cavity
and the insulation layers therebetween, so as to form an ultrasound
vibration element, the substrate is held by a backing with a
low-modulus member therebetween, and a direct voltage and a
alternating voltage are applied between the electrodes to vibrate
the ultrasound vibration element, and wherein a mechanical
impedance by the substrate and the low-modulus member has a
substantially equal value as an acoustic impedance of the
backing.
Inventors: |
Yoshimura; Yasuhiro;
(Kasumigaura, JP) ; Nagata; Tatsuya; (Ishioka,
JP) ; Sako; Akifumi; (Kashiwa, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Yoshimura; Yasuhiro
Nagata; Tatsuya
Sako; Akifumi |
Kasumigaura
Ishioka
Kashiwa |
|
JP
JP
JP |
|
|
Assignee: |
HITACHI MEDICAL CORPORATION
Tokyo
JP
|
Family ID: |
45605271 |
Appl. No.: |
13/817541 |
Filed: |
August 19, 2011 |
PCT Filed: |
August 19, 2011 |
PCT NO: |
PCT/JP2011/068811 |
371 Date: |
February 19, 2013 |
Current U.S.
Class: |
600/443 ;
600/472 |
Current CPC
Class: |
G10K 11/002 20130101;
A61B 8/4444 20130101; A61B 8/4494 20130101; G01N 29/2456 20130101;
B06B 1/0292 20130101; A61B 8/0891 20130101; A61B 8/0883 20130101;
A61B 8/14 20130101 |
Class at
Publication: |
600/443 ;
600/472 |
International
Class: |
A61B 8/00 20060101
A61B008/00; A61B 8/14 20060101 A61B008/14; A61B 8/08 20060101
A61B008/08 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 20, 2010 |
JP |
2010-184882 |
Claims
1. An ultrasound probe comprising an ultrasound vibration element
constituted on a substrate by a cavity, by insulation layers with
the cavity interposed therebetween, and by an upper layer electrode
and a lower layer electrode with the cavity and the insulation
layers interposed therebetween, the substrate being supported by a
backing with a low-modulus member interposed therebetween, the
ultrasound vibration element being vibrated by application of a
direct-current voltage and an alternate-current voltage between the
electrodes; wherein the backing has an acoustic impedance falling
within .+-.1 MRayls (10.sup.6 kg/m.sup.2 s) of a mechanical
impedance formed by the substrate and the low-modulus member.
2. The ultrasound probe according to claim 1, wherein the
mechanical impedance is formed by the mass of the substrate and by
a spring constant of the low-modulus member.
3. The ultrasound probe according to claim 1, wherein the
mechanical impedance formed by the substrate and the low-modulus
member is made to have substantially the same value as the acoustic
impedance of the backing.
4. The ultrasound probe according to claim 1, wherein, if Zx
denotes the mechanical impedance formed by the mass of the
substrate and by the spring constant of the low-modulus member and
Zy represents the value of the acoustic impedance of the backing,
then the values Zx and Zy are made to fall within ranges meeting
the following three expressions: Zx.ltoreq.7.4 MRayls (10.sup.6
kg/m.sup.2 s), Zy.ltoreq.8.3 MRayls (10.sup.6 kg/m.sup.2 s), and
Zy.gtoreq.0.883Zx-0.532 MRayls (10.sup.6 kg/m.sup.2 s).
5. The ultrasound probe according to claim 1, wherein the substrate
is a silicon substrate.
6. The ultrasound probe according to claim 5, wherein the silicon
substrate has a thickness of 50 .mu.m or less.
7. The ultrasound probe according to claim 1, wherein the backing
on a long axis side thereof has a smaller linear expansion
coefficient than on a minor axis side thereof.
8. The ultrasound probe according to claim 1, wherein the backing
is a resin containing carbon fiber.
9. The ultrasound probe according to claim 8, wherein the resin is
added with particles of silica, tungsten or the like subject to
varying densities in low thermal expansion.
10. The ultrasound probe according to claim 1, wherein
approximately half the resonance frequency of mechanical vibrations
of the substrate and the low-modulus member is set as the center
frequency of ultrasonic drive.
11. The ultrasound diagnostic device for obtaining an ultrasound
diagnostic image of a test object using an ultrasound probe
according to claim 1.
Description
TECHNICAL FIELD
[0001] This invention relates to an ultrasound probe for acquiring
diagnostic images and an ultrasound diagnostic device using that
ultrasound probe.
BACKGROUND ART
[0002] An example of the conventional ultrasound probe in the field
of inspecting a test object with ultrasound is disclosed in Patent
Literature 1 cited below. This invention is structured with a gap,
insulation layers, and electrodes formed on a silicon substrate. A
damping material having substantially the same acoustic impedance
as that of the silicon substrate is introduced into the opposite
surface of the substrate. A DC voltage is applied between the
electrodes and the silicon substrate so as to reduce the gap to a
predetermined position. In this structure, an AC voltage is further
applied in a manner contracting or expanding the gap in order to
transmit ultrasound. The ultrasound probe also has the function of
receiving the reflected ultrasound from the test object so as to
detect a capacitance change between the electrodes and the silicon
substrate. Here, the damping material plays the role of reducing
ultrasonic wave reflection during transmission and reception. A
specific damping material is prepared by mixing epoxy resin and
tungsten particles in varying blending ratios, whereby the acoustic
impedance of the material is adjusted to that of the silicon
substrate.
[0003] Also, Patent Literature 2 cited below discloses an
ultrasound probe having a piezoelectric element disposed on an
acoustic backing with an acoustic impedance of 1.3 to 6 MRayls. The
acoustic backing is described as a composite material mixed with
zinc oxide fiber.
[0004] Furthermore, Patent Literature 3 cited below describes a
CMUT (Capacitive Micromachined Ultrasonic Transducer) chip bonded
with a backing to provide short pulses, i.e., a wideband ultrasonic
waveform suitable for high-resolution use.
CITATION LIST
Patent Literature
[0005] Patent literature 1: U.S. Pat. No. 6,714,484B2 [0006] Patent
literature 2: Japanese Patent Application Laid-Open Publication No.
2008-118212 [0007] Patent literature 3: Japanese Patent Application
Laid-Open Publication No. 2008-119318
SUMMARY OF INVENTION
Technical Problem
[0008] The ultrasound probe for use with an ultrasound diagnostic
device transmits ultrasound waves to a test object, receives the
ultrasonic waves reflected from the test object, and turns the
received ultrasonic waves into an image. Ultrasonic waves have the
nature of reflecting from an interface between materials having
different acoustic impedances. For this reason, a drop in image
quality can result from ultrasonic waves reflecting from interfaces
between an ultrasound transmission/reception device constituting
the ultrasound probe, an acoustic lens disposed on the front side
of the device, and the backing on the back side. The primary method
of reducing reflection on the front side involves providing between
the acoustic lens and the ultrasound transmission/reception device
a matching layer having an intermediate acoustic impedance. Then
there exists the frequently adopted technique of attenuating
ultrasonic waves reaching the back side in the backing by making
the acoustic impedance of the backing equal to that of the
ultrasound transmission/reception device. However, the reflection
from the back side stems from factors intrinsic to the CMUT, as
will be described below. This has made it difficult for the
conventional methods of equalizing acoustic impedances to reduce
the ultrasonic wave reflection.
[0009] An object of this invention is to seek the cause of the
above-mentioned back side reflection and take appropriate
countermeasures to reduce the reflection of ultrasonic waves
emitted from the ultrasound transmission/reception device to the
back side, thereby obtaining high-quality diagnostic images.
[0010] More specifically, it has been found that in the
CMUT-equipped probe, vibrations applied to a membrane over a cavity
are propagated to the silicon substrate via a narrow rim supporting
the membrane and that while being dispersed cylindrically within
the silicon substrate, the vibrations engender reflection. This
invention thus aims to provide a structure for preventing the
acoustic reflection on the back side over the wideband.
Solution to Problem
[0011] In achieving the foregoing object of this invention and
according to one aspect thereof, there is provided an ultrasound
probe including an ultrasound vibration element constituted on a
substrate by a cavity, by insulation layers with the cavity
interposed therebetween, and by an upper layer electrode and a
lower layer electrode with the cavity and the insulation layers
interposed therebetween, the substrate being supported by a backing
with a low-modulus member interposed therebetween, the ultrasound
vibration element being vibrated by application of a direct-current
voltage and an alternate-current voltage between the electrodes.
The backing has an acoustic impedance falling within .+-.1 MRayls
(10.sup.6 kg/m.sup.2 s) of a mechanical impedance formed by the
substrate and the low-modulus member.
[0012] According to another aspect of this invention, there is
provided an ultrasound diagnostic device for obtaining an
ultrasound diagnostic image of a test object using the ultrasound
probe outlined above.
Advantageous Effects of Invention
[0013] This invention provides an ultrasound probe capable of
reducing the reflection of ultrasonic waves emitted from an
ultrasound transmission/reception device to the back side thereof.
The invention further provides an ultrasonic diagnostic device
capable of presenting high-quality diagnostic images using the
ultrasound probe of this invention.
BRIEF DESCRIPTION OF DRAWINGS
[0014] FIG. 1 is a schematic view showing an overall structure of
an ultrasound probe.
[0015] FIG. 2 is a cross-sectional view of an ultrasound
transmission/reception device, a backing, an acoustic lens and the
like.
[0016] FIG. 3 is a perspective view showing the ultrasound
transmission/reception device, the backing, and a flexible
substrate.
[0017] FIG. 4 is a partially enlarged view of the ultrasound
transmission/reception device.
[0018] FIG. 5 is a partial cross-sectional view of a cell of the
ultrasound transmission/reception device.
[0019] FIG. 6 is a cross-sectional view of the backing in the long
axis direction thereof.
[0020] FIG. 7 is an explanatory view explaining the model of
ultrasonic wave reflection.
[0021] FIG. 8 is a graphic representation showing the relationship
between the thickness of the ultrasound transmission/reception
device substrate and reflection factors.
[0022] FIG. 9 is a graphic representation showing the relationship
between frequencies and reflection factors from analysis of the
reflection on the back side of the ultrasound
transmission/reception device.
[0023] FIG. 10 is a graphic representation showing the relationship
between frequencies and phases from analysis of the reflection on
the back side of the ultrasound transmission/reception device.
[0024] FIG. 11 is a contour drawing of reflection factors at 1/4 of
the resonance frequency f.sub.0 between the backing and the
low-modulus member as a result of analysis by the finite element
method.
[0025] FIG. 12 is a contour drawing of reflection factors at 1/2 of
the resonance frequency f.sub.0 between the backing and the
low-modulus member as a result of analysis by the finite element
method.
[0026] FIG. 13 is a contour drawing of reflection factors at 3/4 of
the resonance frequency f.sub.0 between the backing and the
low-modulus member as a result of analysis by the finite element
method.
[0027] FIG. 14 is a contour drawing summarizing the maximum
reflection factors shown in FIGS. 11, 12 and 13 as a result of
analysis by the finite element method.
DESCRIPTION OF EMBODIMENTS
[0028] A preferred embodiment of this invention is explained below
in reference to FIGS. 1 through 14.
[0029] FIG. 1 shows an overall structure of an ultrasound probe 1
furnished with an ultrasound transmission/reception device
(ultrasound vibration element) 2. The ultrasound probe 1 is used at
medical institutions in examining the human organism (examination
of the heart, blood vessels, and other circulatory organs;
abdominal examination and the like). The ultrasound probe 1 has a
backing 3 tipped with the ultrasound transmission/reception device
2. A flexible substrate 4 having wiring 92 leading to a connector
91 is bonded to the ultrasound transmission/reception device 2 by
wire bonding. The connector 91 is connected to a circuit substrate
97, and a connecting terminal 98 of the circuit substrate 97 is
connected to an ultrasound diagnostic device. The ultrasound
diagnostic device drives the ultrasound transmission/reception
device 2 by giving it an electrical signal and turns a signal
representing the reflected ultrasonic waves from the test object
into an image. The surface of the ultrasound transmission/reception
device 2 is furnished with an acoustic lens 94 of silicon resin
designed to focus the ultrasonic waves generated by the ultrasound
transmission/reception device 2 in the direction of the test
object. The ultrasound transmission/reception device 2 transmits
and receives ultrasonic waves to and from a test object 95 such as
the human organism through the acoustic lens 94.
[0030] FIG. 2 is a cross-sectional view showing the acoustic lens
94, the ultrasound transmission/reception device (ultrasound
vibration element) 2, and the backing 3 of the ultrasound probe 1
indicated in FIG. 1 along with surrounding constructs. The
ultrasound transmission/reception device 2 is disposed on the
backing 3 with a resin 45 interposed therebetween. The flexible
substrate 4 for communicating ultrasound transmission/reception
signals with the substrate (not shown) is also fixed to the backing
3 with a resin 46 interposed therebetween. The ultrasound
transmission/reception device 2 and the flexible substrate 4 are
bonded by wire bonding using a wire 42. The wire 42 and the
surroundings of where it is bonded are sealed by a sealing resin
47. The sealing has the effect of securing the wire 42 and
preventing electro-migration caused by application of a drive
voltage. Onto these constructs, the acoustic lens 94 is fixedly
bonded using a resin 41. Also, these constructs are structured to
be housed in a case 43. The gap between the case 43 and an acoustic
lens 2 is filled with a resin 44.
[0031] FIG. 3 shows the ultrasound transmission/reception device 2,
the backing 3, and the flexible substrate 4 explained in reference
to FIG. 1. The long side direction of the ultrasound
transmission/reception device 2 and backing 3 is indicated as a
long axis direction L and the short side direction thereof as a
minor axis direction M. The ultrasound transmission/reception
device 2 is bonded onto the backing 3 by resin. The ultrasound
transmission/reception device 2 is connected with the flexible
substrate 4 for power supply and signal transmission. The flexible
substrate 4 and backing 3 are bonded together, and electrode pads
(not shown) of the ultrasound transmission/reception device 2 and
wiring pads (not shown) of the flexible substrate 4 are
interconnected (not shown) by wire bonding as indicated in FIG.
2.
[0032] FIG. 4 is a partially enlarged view of the ultrasound
transmission/reception device 2. The ultrasound
transmission/reception device 2 is composed of a plurality of cells
21 arranged in highly concentrated fashion. FIG. 4 indicates a long
side direction A and a short side direction B of each cell 21, a
cell spacing C, and a cell pitch D. The long axis direction L shown
in FIG. 3 is also indicated. A plurality of cells 21 are arranged
into a channel, and the wiring 92 is connected to each of the
channels for control over the transmission and reception of
ultrasonic waves.
[0033] FIG. 5 is a cross-sectional view taken on line E-E in FIG. 4
covering two cells 21 of the ultrasound transmission/reception
device 2, the view indicating the backing 3 as well. The cell 21 is
made up of a substrate 22 serving as a base substrate, insulation
films 26a through 26e; a lower layer electrode 23 and an upper
layer electrode 25 constituting parallel plate electrodes, and a
cavity 24 between the electrodes. The wall between the cells forms
a rim 27. A low-modulus member 5 is interposed between each cell 2
and the backing 3. The low-modulus member 5 is a resin that bonds
the cells 2 to the backing 3. That is, disposed on the substrate 22
are the cavity 24, insulation layers 26b and 26c sandwiching the
cavity 24, and upper layer electrode 25 and lower layer electrode
23 sandwiching the cavity and insulation layers making up an
ultrasound vibration element, with the substrate 22 supported by
the backing 3 with the low-modulus member 5 interposed
therebetween. FIG. 5 also shows the long axis direction L indicated
in FIGS. 3 and 4. Although the material of the substrate 22 should
preferably be silicon, a low thermal expansion material such as
glass or ceramic may also be used instead. As the low-modulus
member 5, a material such as epoxy or rubber is preferred.
[0034] When a DC voltage is applied between the lower layer
electrode 23 and upper layer electrode 25 and supplemented with a
pulse voltage (AC voltage), Coulomb's force causes a membrane
composed of the insulation layers 26c, 26d, 26e and the upper layer
electrode 25 to vibrate and emit ultrasonic waves. When reflected
waves from the test object 95 enter the cell, the membrane
vibrates, changing the distance between the lower layer electrode
23 and the upper layer electrode 25. This generates a displacement
current that is converted to a received electrical signal. When a
force is applied to the membrane during such transmission and
reception, the rim 27 supporting the membrane is subjected to the
force whereby ultrasonic waves are propagated to the substrate
22.
[0035] FIG. 6 is a cross-sectional view taken on line F-F in FIG. 3
of the backing 3. Carbon fiber 32 is mixed into the resin material.
Preferably, the mixed carbon fiber 32 should be oriented in the
long axis direction L of the backing 31, at 30 degrees or less
relative to the long axis direction L. The mixed carbon fiber 32
may also be in parallel with the long axis direction L. In the
latter case, the carbon fiber 32 is approximately perpendicular to
the minor axis direction M. The array direction of the carbon fiber
32 follows the short side direction B of the cell 2. When the
carbon fiber 32 of a low thermal expansion coefficient is mixed
into the backing in the direction shown in FIG. 6, it is possible
significantly to reduce the thermal expansion coefficient of the
backing 3 in the long axis direction L thereof compared with the
thermal expansion coefficient in the minor axis direction M. For
example, the thermal expansion coefficient of about 100 ppm of a
resin may be reduced to 1 to 20 ppm by suitably setting the
blending ratio. Preferably, the blending ratio of the carbon fiber
32 should be 20 to 50 percent by volume. The carbon fiber 32 should
preferably be 10 .mu.m to 10 mm in length. The diameter of the
carbon fiber should preferably be 2 to 100 .mu.m. Thus when the
thermal expansion coefficient of the backing in the long axis
direction L thereof in the short side direction B of the cell 2 is
made close to the thermal expansion coefficient of the substrate
22, it is possible to alleviate thermal stress stemming from the
bonding of the ultrasound transmission/reception device 2 and
backing 3 or from the process of fabrication. Distortion in the
short side direction B of the cell 2 can significantly affect the
performance of ultrasound transmission and reception, i.e.,
variance of the channels arrayed in the long axis direction L. For
that reason, the distortion due to thermal stress should preferably
be as small as possible. Thus the structure of this invention
lowers the thermal expansion coefficient of the backing 3 by
arraying the carbon fiber 32 of the backing 3 in the short side
direction B of the cell 2. Furthermore, it is preferred to add
particles of silica 33, tungsten 34 or the like of low thermal
expansion varying in density in order to adjust thermal expansion
coefficient and acoustic impedance. Where particles of low thermal
expansion are added, the thermal expansion coefficient in the minor
axis direction M can also be reduced, which contributes to
alleviating thermal stress.
[0036] Explained next is the reflection of ultrasonic waves on the
back side of the cell 21. The ultrasonic waves emitted from the
cell 21 shown in FIG. 5 are released also in the back side
direction of the cell 21, i.e., opposite to the direction of the
test object 95 (in the frontal direction) through the rim 27. When
the ultrasonic waves propagated in the back side direction are
reflected and returned in the frontal direction, pulse decay time
is prolonged and the waveform in effect is worsened. Also, when the
ultrasonic waves are reflected from, say, the subcutaneous fat of
the test object 95, returned to the cell 21, and then reflected by
the back side thereof before being released in the frontal
direction, the cell 21 may detect not only the ultrasonic wave echo
originally reflected from the test object organism but also the
ultrasonic waves reflected from the back side as mentioned above.
This can cause virtual images such as ghosts to appear in the
diagnostic image or can reduce the resolution of the latter image.
It is thus necessary to minimize the reflection from the back side.
Empirically, the reflection factor of the reflection from the back
side needs to be -10 dB (31%) or less in order to prevent
deterioration of the diagnostic image caused by the back side
reflection.
[0037] In order to examine the cause of the reflection from the
back side of the cell 21, the inventors analyzed by the finite
element method the reflection of ultrasonic waves entered at 10 MHz
from a narrow rim 27 shown in FIG. 7 into the substrate 22 made of
silicon. In this case, the bottom side of the substrate 22 was
regarded as an acoustic absorption boundary. FIG. 8 shows the
relationship between the thickness of the substrate 22 and
reflection factors. Despite the absence of a reflective interface,
the reflection was found to be abruptly more pronounced the thicker
the substrate 22. In connection with audio equipment such as
speakers, a phenomenon is known in which, if vibrating parts are
small compared with the wavelength involved, sounds fail to
propagate when sound pressure and volume velocity become out of
phase due to the spherical propagation of the waves. Because the
wavelength of the substrate 22 made of silicon was as large as
approximately 8,500 .mu.m at 10 MHz compared with the size of the
rim 27 being several .mu.m, the ultrasonic waves were found to be
reflected as they spread cylindrically (indicated by broken lines)
in the arrowed directions within the substrate 22 shown in FIG. 7.
It was found that the conventional method of getting the acoustic
impedance of the substrate to coincide with that of the backing is
not effective for the CMUT that necessitates using the rim 27. The
thickness of the silicon substrate should preferably be 50 .mu.m or
less.
[0038] Studies were conducted to reduce the reflection from the
back side of the cell 21. The affectors involved were the
thickness, Young's modulus, and density of silicon as the
substrate; Young's modulus and density of the backing, width B of
the cell 21, width C of the rim 27 interposed between the cells,
and thickness of the low-modulus member 5. The preferred ranges of
these affectors were calculated through analysis by the finite
element method.
[0039] FIG. 9 shows an example of the results of the analysis,
plotting the relationship between reception frequency f and the
reflection coefficient R of the back side. The cell width B was set
to be 25 .mu.m, and the thickness of the substrate 22 made of
silicon and that of the low-modulus member 5 were set to be 50
.mu.m and 10 .mu.m respectively, with the elastic modulus of the
backing 3 varied. In FIG. 9, a line 6-1 stands for a reference
elastic modulus of the backing 3, a line 6-2 for twice the
reference elastic modulus, a line 6-3 for 0.5 times the reference
elastic modulus, and a line 6-4 for 0.25 times the reference
elastic modulus. Varying Young's modulus of the backing corresponds
to multiplying the acoustic impedance of the backing by square
roots as stated below. Since the loss caused by reflection should
fall within -10 dB, i.e., within 31%, plotting a line of -10 dB in
FIG. 9 reveals that the case of the line 6-3 (indicated by a hollow
circle o) provides the widest range of frequencies and should be
preferred under these conditions.
[0040] FIG. 10 shows the relationship between the frequency f and
phase .theta. in the results indicated in FIG. 9. A line 7-1 in
FIG. 10 corresponds to the line 6-1 in FIG. 9, a line 7-2 to the
line 6-2, a line 7-3 to the line 6-3, and a line 7-4 to the line
6-4. Whereas the line 7-3 indicates the result under the condition
represented by the line 6-3, there are many ranges where the phase
is zero, which signifies a gentle change (indicated by a hollow
circle o). This is probably attributable to the out-of-phase state
being alleviated thanks to the cylindrical wave diffusion from the
rim caused by mechanical vibrations of the silicon substrate 22 and
the low-modulus member 5, whereby sounds toward the back side may
effectively be propagated to the backing. At this point, the
resonance frequency of mechanical vibrations between the silicon
substrate 22 and the low-modulus member 5 is approximately 10 MHz.
The reflection can then be lowered over a wideband centering on
about half that resonance frequency. That is, a frequency
approximately half the resonance frequency of mechanical vibrations
between the substrate and the low-modulus member need only be set
as the center frequency for ultrasonic drive. The short pulses over
a wideband, characteristic of the CMUT, are distorted in waveform
and deteriorate when the reflection is reduced over a narrowband.
It was thus found that the reflection can be lowered without
deterioration of short pulses when the reflection over the wideband
is reduced by suitably setting the mechanical vibrations of the
substrate 22 and the low-modulus member 5 and the acoustic
characteristics of the backing 3.
[0041] The vibrations of the substrate 22 and the low-modulus
member 5 are characterized by mechanical impedance Zm of
one-degree-of-freedom vibrations. The mechanical impedance is
defined by the mathematical expression 1 shown below in which M
denotes the mass per unit area of the substrate 22 and k stands for
the spring constant per unit area of the low-modulus member 5. In
this case, the mass M per unit area is obtained from M=t.rho. where
t stands for the thickness and .rho. stands for the density of the
substrate 22. The spring constant k is obtained using the
mathematical expression 2 below in which E denotes Young's modulus,
.nu. represents Poisson's ratio, and d stands for the thickness of
the low-modulus member 5.
Zm = Mk [ Math . 1 ] k = 1 - v ( 1 + v ) ( 1 - 2 v ) E d [ Math . 2
] ##EQU00001##
[0042] The following mathematical expression 3 gives the resonance
frequency f.sub.0 in effect when the substrate 22 is regarded as
the mass M and the low-modulus member 5 as the spring constant
k:
f 0 = 1 2 .pi. k M [ Math . 3 ] ##EQU00002##
[0043] The following mathematical expression 4 gives the acoustic
impedance Z of the backing 3 in effect when Eb stands for Young's
modulus, .rho..sub.b for the density, and .nu..sub.b for Poisson's
rate of the backing:
Z = E b .rho. b 1 - v b ( 1 + v b ) ( 1 - 2 v b ) [ Math . 4 ]
##EQU00003##
[0044] Under the conditions of FIGS. 9 and 10, the resonance
frequency f.sub.0 of one-degree-of-freedom vibrations is
approximately 10 MHz. Above the resonance frequency, the reflection
increases abruptly; at low frequencies, low reflection is available
over a flat wideband. For this reason, half the
one-degree-of-freedom resonance frequency f.sub.0 may be set as the
center frequency, so that low reflection may be made available
within the range of 1/4 to 3/4 times the frequency f.sub.0
constituting a frequency band of 100%.
[0045] FIGS. 11 through 14 are contour drawings of reflection
factors obtained through analyses performed in like manner by the
finite element method, the reflection factors plotting the acoustic
impedance Z of the backing versus the mechanical impedance Zm where
the substrate 22 is regarded as the mass and the low-modulus member
as the spring constant. In these drawings, dashed lies denote the
condition under which the mechanical impedance is equal to the
acoustic impedance.
[0046] FIG. 11 shows contours of the reflection factors resulting
from analysis by the finite element method at a frequency 1/4 times
the resonance frequency f.sub.0 of the backing 3 and the
low-modulus member 5.
[0047] FIG. 12 shows contours of the reflection factors resulting
likewise from analysis by the finite element method at a frequency
half the resonance frequency f.sub.0 of the backing 3 and the
low-modulus member 5.
[0048] FIG. 13 shows contours of the reflection factors resulting
likewise from analysis by the finite element method at a frequency
3/4 times the resonance frequency f.sub.0 of the backing 3 and the
low-modulus member 5.
[0049] FIG. 14 is a contour drawing summarizing the maximum
reflection factors shown in FIGS. 11, 12 and 13, the contours
showing regions where the reflection factors are small over a
wideband, the regions being preferred for reducing the back side
reflection.
[0050] A straight line A in FIG. 14 denotes a line along which the
mechanical impedance is equal to the acoustic impedance of the
backing. The most preferred range is included over this line. And
straight lines B and C with the straight line A disposed
therebetween indicate the range where the acoustic impedance reads
.+-.1 MRayls. This range flanked by the straight lines may be
stipulated as a range in which the mechanical impedance is
substantially equal to the acoustic impedance of the backing,
whereby the back side reflection can be reduced. That is, the
acoustic impedance of the backing may be set to be within .+-.1
MRayls (10.sup.6 kg/m.sup.2 s) of the mechanical impedance formed
by the substrate and the low-modulus member. Also, the mechanical
impedance formed by the substrate and the low-modulus member may be
set to be approximately equal in value to the acoustic impedance of
the backing.
[0051] Also in FIG. 14, the ranges where the reflection factors of
-10 dB or less are available occur where the mechanical impedance
formed by the low-modulus member 5 and substrate 22 is 7.4 MRayls
or less and where the acoustic impedance of the backing 3 is 8.3
MRayls or less. The region where the reflection factors are -10 dB
or less is stipulated as the region enclosed by straight lines D, E
and F. If Zx denotes the value of the mechanical impedance formed
by the mass of the substrate and the spring constant of the
low-modulus member and Zy represents the value of the acoustic
impedance of the backing, the region in question is defined as one
that satisfies the mathematical expressions 5, 6 and 7 below at the
same time, i.e., the range in which the back side reflection may be
reduced.
Zx.ltoreq.7.4 MRayls (10.sup.6 kg/m.sup.2 s) [Math. 5]
Zy.ltoreq.8.3 MRayls (10.sup.6 kg/m.sup.2 s) [Math. 6]
Zy.gtoreq.0.883Zx-0.532 MRayls (10.sup.6 kg/m.sup.2 s) [Math.
7]
[0052] According to the above-described embodiment, for the
CMUT-equipped ultrasound probe, the range of values of the
mechanical impedance formed by the mass of the substrate and the
spring constant of the low-modulus member and the range of values
of the acoustic impedance of the backing are stipulated in such a
manner as to lower the reflection of ultrasonic waves released
toward the back side, whereby high-quality diagnostic images are
acquired.
REFERENCE SIGNS LIST
[0053] 1 Ultrasound probe [0054] 2 Ultrasound
transmission/reception device [0055] 3 Backing [0056] 4 Flexible
substrate [0057] 5 Low-modulus member [0058] 21 Cell [0059] 22
Substrate [0060] 23 Lower layer electrode [0061] 24 Cavity [0062]
25 Upper layer electrode [0063] 26a, 26b, 26c, 26d, 26e Insulation
films [0064] 27 Rim [0065] 31 Backing [0066] 32 Carbon fiber [0067]
33 Silica [0068] 34 Tungsten [0069] 41 Resin [0070] 42 Wire [0071]
43 Case [0072] 44, 45, 46 Resins [0073] 47 Sealing resin [0074] 91
Connector [0075] 92 Wiring [0076] 94 Acoustic lens [0077] 95 Test
object [0078] 97 Circuit substrate [0079] 98 Connecting
terminal
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